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Accumulated Other Comprehensive Income (Details) - CAD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Components and changes in accumulated other comprehensive Income (loss) |  |  |  |  |
Accumulated other comprehensive income, beginning of the period |  | $ 454 |  |  |
Currency translation adjustment |  |  |  |  |
Losses on translating net assets of foreign operations, net of reclassifications to net earnings, net of tax |  |  |  | $ 38 |
Gains (losses) on financial instruments designated as hedges of foreign operations, net of tax |  | 11 |  | 21 | $ (41)
Cash flow hedges |  |  |  |  |
Gains (losses) on derivatives designated as cash flow hedges, net of tax | [1] | 20 |  | 61 | (8)
Other |  |  |  |  |
Intercompany FVOCI investments |  | 0 |  | 0 |
Accumulated other comprehensive income, end of the period |  | 302 |  | 454 |
Net impact related to cash flow hedges |  | (23) |  | 6 | (12)
Income tax relating to net actuarial gains (losses) |  | 3 |  | 7 | (5)
Reserve of exchange differences on translation |  |  |  |  |
Components and changes in accumulated other comprehensive Income (loss) |  |  |  |  |
Accumulated other comprehensive income, beginning of the period |  | (21) |  | 17 |
Currency translation adjustment |  |  |  |  |
Losses on translating net assets of foreign operations, net of reclassifications to net earnings, net of tax |  | (11) |  | (59) |
Gains (losses) on financial instruments designated as hedges of foreign operations, net of tax |  | 11 |  | 21 |
Other |  |  |  |  |
Accumulated other comprehensive income, end of the period |  | (21) |  | (21) | 17
Reserve of cash flow hedges |  |  |  |  |
Components and changes in accumulated other comprehensive Income (loss) |  |  |  |  |
Accumulated other comprehensive income, beginning of the period |  | 527 |  | 508 |
Cash flow hedges |  |  |  |  |
Gains (losses) on derivatives designated as cash flow hedges, net of tax |  | (91) |  | 19 |
Other |  |  |  |  |
Accumulated other comprehensive income, end of the period |  | 436 |  | 527 | 508
Net impact related to cash flow hedges |  | 23 |  | 6 |
Reserve of remeasurements of defined benefit plans |  |  |  |  |
Components and changes in accumulated other comprehensive Income (loss) |  |  |  |  |
Accumulated other comprehensive income, beginning of the period |  | (55) |  | (29) |
Employee future benefits |  |  |  |  |
Net actuarial gains (losses) on defined benefit plans, net of tax |  | (11) |  | (26) |
Other |  |  |  |  |
Accumulated other comprehensive income, end of the period |  | (66) |  | (55) | (29)
Income tax relating to net actuarial gains (losses) |  | (3) |  | (7) |
Other reserves |  |  |  |  |
Components and changes in accumulated other comprehensive Income (loss) |  |  |  |  |
Accumulated other comprehensive income, beginning of the period |  | 3 |  | (15) |
Other |  |  |  |  |
Change in ownership of TransAlta Renewables |  | 0 |  | 1 |
Intercompany FVOCI investments |  | (50) | [2] | 17 |
Accumulated other comprehensive income, end of the period |  | $ (47) |  | $ 3 | $ (15)
[1] | Net of income tax expense of $8 million for the year ended Dec. 31, 2020 (2019 —$16 million expense, 2018 — $1 million recovery).
[2] | Refer to Note 29 for details on components of, and changes in, accumulated other comprehensive income (loss).

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